AnMal one way to get a fairly good transparent bevel is to use the filter edge detect sobel. Here is an Image created using this technique
![Image](http://www.gimpchat.com/files/2144_Ubuntu%20Community.jpg)
The bevel is all on a seperate layer and starts at step 16 of this tutorial
Cubism
1. Open new layer A3 set to landscape. Go Layer>Transparency>Add Alpha Channel.
2. Go Filter>Render>Clouds>Plasma set Turbulence to 7 check Randomize click OK.
3. Go Layers>Transform>Offset set Y axis to 49 Check wrap around and click OK.
4. Go Filter>Blur>Pixelise set Height and Width to 155 click OK.
5. Go Layers>Transform>Offset set Y axis to 49 Check wrap around and click OK.
6. Duplicate Layer and Go>Colours>desaturate Check Luminosity and click OK.
7. Duplicate Grey scale Layer and Base Layer Again. Reposition Layers to go Grey, Colour, Grey & Colour.
8. Change top Grey Layer Mode to Dodge and the Second Grey Layer Mode to Burn.
9. Select the top Grey Layer and Go Layers>Merge Down
10. Select the Bottom Grey Layer and Go Layers>Merge down
11. Select the Top Layer and Change Layer Mode to Grain Merge, Go Layer>Merge Down
12. Go>Colours>desaturate Check Luminosity and click OK.
13. Go>Colours>Posturize set Posturize Level to 36 then Click OK.
14. Select a desired Palette from Palettes Dialogue window.
15. Go>Colours>Map>Palette Map
16. Duplicate Layer Twice, Select Middle Layer.
17. Go>Filter>Edge Detect>Sobel check Sobel Horizontal Uncheck Sobel Vertical Check Keep sign of Result (one direction Only)
18. Select top Layer
19. Go>Filter>Edge Detect>Sobel uncheck Sobel Horizontal check Sobel Vertical Check Keep sign of Result (one direction Only)
20. Select the Top Layer and Change Layer Mode to Grain Merge, Go Layer>Merge Down
21. Go Select by Colour, Uncheck all Boxes set Threshold to 0 and Select by to Composite
22. Click on the large grey area of Image.
23. Go Edit>Clear then Go Select>None.
24. Go>Colours>desaturate Check Luminosity and click OK.
25. Turn off all other layers except top Layer. Select Top Layer, change Layer Mode to Overlay and Duplicate Layer 5*.
26. Right Click on one of the layers and select Merge Visible Layers click OK.
27. Turn off all other layers except top Layer. Select Top Layer, change Layer Mode to Overlay and Duplicate Layer 5*.
28. Right Click on one of the layers and select Merge Visible Layers click OK.
29. Go Colours Invert, Duplicate Layer.
30. Select Top Layer, Go Layers>Transform>Offset set Z Axis to 1, set Y axis to1 Check Make Transparent and click OK.
31. Select Next Layer Down, Go Layers>Transform>Offset set Z Axis to -1, set Y axis to-1 Check Make Transparent and click OK.
32. Go>Layer>Merge Down
33. Duplicate Layer, Select top Layer.
34. Go>Filters>Blur>Gaussians Blur set Height to 25 and width to 25 Click OK.
35. Turn off all other layers except top Layer. Select Top Layer, Duplicate Layer 5*.
36. Right Click on one of the layers and select Merge Visible Layers click OK and change Layer Mode to Overlay.
37. Select the Middle Layer.
38. Go>Filters>Blur>Gaussians Blur set Height to 5 and width to 5 Click OK, change Layer Mode to Overlay.
If you want to see some more examples the go here
He4rty's Cubism Gallery